1. Field of the Invention
The present invention relates to an ionizer for alternately generating positive and negative ions, a static charge eliminating system having such an ionizer, an ion balance adjusting method for adjusting an ion balance of positive ions and negative ions, and a workpiece static charge eliminating method to which the ion balance adjusting method is applied.
2. Description of the Related Art
Heretofore, it has widely been known to neutralize positive or negative electric charges that have charged a workpiece, to thereby eliminate static charges from the workpiece, by releasing positive and negative ions toward the workpiece from an ionizer. In U.S. Pat. No. 4,630,167, U.S. Pat. No. 4,809,127, Japanese Patent Publication No. 06-047006, and Japanese Laid-Open Patent Publication No. 2007-149419, adjustment of the balance (ion balance) between a positive ion amount and a negative ion amount inside of a space (static charge eliminating space) where static charge removal on the workpiece is performed has been proposed, by means of an ionizer which alternately carries out generation of positive ions and negative ions.
With the aforementioned ionizer, as a result of corona discharge occurring at a distal end side of an electrode caused by application of positive or negative voltages with respect to the electrode, positive ions or negative ions are generated inside the static charge eliminating space. In this case, as confirmed by the present applicants, the density of ozone (ozone density) generated inside the static charge eliminating space caused by corona discharge is greater when a negative voltage is applied with respect to the electrode than when a positive voltage is applied with respect to the electrode (see FIGS. 10A and 10B). Owing thereto, metals (for example, the electrode, etc.) utilized in the ionizer become oxidized and corroded as a result of the generation of ozone by application of the negative voltage. Alternatively, the user of the ionizer tends to sense the ozone as an unusual odor.
With respect to these problems, by decreasing the absolute value of the negative voltage applied to the electrode, the ozone density can be reduced (see, FIG. 10A). However, if the absolute value of the negative voltage is reduced, the field intensity at the distal end side of the electrode decreases and the generated amount of negative ions is reduced, such that the ion balance of positive ions and negative ions is subject to deterioration. Therefore, the time required to eliminate static charges from the workpiece (hereinafter referred to as the “charge removal time”) becomes considerably longer (see, FIG. 11A). Accordingly, the above-mentioned problems cannot be overcome and resolved simply by reducing the absolute value of the negative voltage.